The invention relates to a method for the extraction of a substance from a starting material, wherein the starting material is brought together with an extraction fluid in an extruder. The starting material is moved along an extrusion channel in a conveyor device. The extruder contains at least two pressure-raising elements which are spaced apart, with a low pressure zone between the elements and a high pressure zone located downstream thereof.
In the foodstuffs industry, organic waste streams and specific product streams frequently contain high value components. These components are usually extracted from the product streams batchwise. Since the starting material is usually pressed, followed by precipitation, washing out by means of boiling of by means of a belt press, large quantities of extraction fluid are used.
It is also known to produce bleached chemical-mechanical wood pulp from wood chips by feeding the wood chips through a twin screw extruder such as that manufactured by Clextra under the trade name BIVIS. In this process a number of screw elements with opposing pitch (reverse screw elements) are used in the casing of the extruder to form high pressure zones for intensive kneading of the product. Chemicals such as NaOH and NaHSO3 as well as steam are injected into the casing of the extruder and discharged from the extruder in a direction concurrent with the direction of transport of the starting material.
A process of this type has the disadvantage that it is not optimum for extraction and that the starting material can be forced out of the casing of the extruder under pressure at the tapping points. As a result, the pressures which can be used are limited and the concentrations of substances which can be extracted in an extraction process are usually relatively low.
FR 2 619 514 describes an extraction process using a twin screw extruder, wherein extraction takes place at relatively low pressures in a number of concurrent processes connected in series.
WO 90/10484 discloses an extraction method in which the starting material is moved by means of a single screw extruder in a direction countercurrent to a solvent. With the known apparatus having a single screw, expression of the starting material with extractant in the compression zone is possible only to a limited extent since if too much fluid is used no further solid is transported. Extraction under high pressure is also not possible in the known apparatus, while the flow of the starting material and the extractant are highly integrated, so that the extraction is relatively inefficient.
It is therefore an aim of the present invention to provide an extraction process in which relatively high pressures can be used, very good mixing of the starting material and the extraction fluid occurs, a high yield is achieved and the risk of the starting material being forced out of the casing through the discharge openings is reduced.
A further aim of the present invention is to provide an extraction process which can be operated continuously and various substances can be extracted from a starting material with high selectivity.
To this end the method according to the present invention is characterized in that the extraction fluid is fed to the extruder via a feed opening in or close to the high pressure zone, is then brought to the low pressure zone in a direction countercurrent to the direction of transport of the starting material and is discharged from the extruder via a discharge opening in or close to the low pressure zone. The starting material is continually supplied to the low pressure zone in the direction of transport and continuously discharged from the high pressure zone in the direction of transport.
According to the invention, the extraction fluid is supplied to the extruder and fed in a countercurrent direction over a continuous supply of the starting material through the extrusion channel. Surprisingly, it has been found that the material to be extracted is highly comminuted in the high pressure zones, so that a very high surface area/volume ratio is obtained and very good mixing with the extraction fluid (solvent) occurs. Efficient extraction is possible by means of the countercurrent flow, with a very high extraction yield. By placing the discharge openings for the extraction fluid in or close to the low pressure zones, the pressure in the extraction apparatus is raised to relatively high levels, such as, for example, higher than 10 bar, without the starting material being forced out through the discharge openings. At these high pressures the solubility of the material to be extracted in the extraction fluid is high and extraction can take place efficiently. At the high pressures, the substance to be extracted can be dissolved more rapidly by passing through an arbitrary solvent such as, for example, water or a salt solution in a countercurrent direction. By separate discharge of the extraction fluid from the extruder, while the starting material is transported further along the extrusion channel, a high grade substance, such as an oligomer or polymer condensate, can be obtained efficiently and continuously from a low grade residual stream or waste stream. The extracted, insoluble comminuted residual fraction can be further transported to a second low pressure zone and a second high pressure zone. An extraction fluid once again is supplied, which extraction fluid is discharged close to the second low pressure zone and a further substance can be extracted from thee starting material, with a relatively high selectivity. The properties of the extracted product can be controlled by the selection of the extraction fluid used, the specific pressures and temperatures in the extruder, the starting material/extraction fluid ratio, contact time and the like.
Examples of extracted substances include secondary metabolites, such as odor substances, colorants and flavorings, for example carvone and limonene from caraway seeds, vanilla from vanilla beans, pentosans from wheat waste, pectin from the skins of citrus fruits and inulin from chicory. Furthermore, the method according to the invention can also be used for non-organic materials,- such as the extraction of catalyst residues from polymers. With the method according to the present invention it is also possible advantageously to use an extraction fluid in the supercritical state, such as CO2 at 75 bar and 35xc2x0 C. The supercritical state can be maintained over the entire extraction length.
In one embodiment of the method according to the invention, after the extrusion step the extruded starting material is fed to drying or press means to produce, for example, fiberboard, cattle feed pellets and covering for a stall floor.
The extraction apparatus according to the invention comprises an extruder, for example a multi-screw extruder, with a discharge opening in the wall of the extrusion channel in or close to the low pressure zone for discharging the extraction fluid. The discharge opening is provided with screening means. M A feed opening is made in the wall of the extrusion channel in or close to the high pressure zone located downstream of the low pressure zone.
Preferably, the pitch of the screw in the extruder is relatively large in the low pressure zones and is relatively small in the high pressure zones. In this way the pressure in the extruder can be controlled in a simple manner by adjusting the pitch of the screw. High pressure zones and low pressure zones can also be achieved by locally reversing the pitch of the screw with respect to the pitch of the transport part of the screw.
The extraction apparatus according to the invention is of modular construction and is made up of modules which can be coupled to one another. The modules can be coupled to one another by coupling elements such as external flanges which can be connected to one another. By adjusting the number of modules, various substances can be extracted from the starting material as required with the aid of a number of different extraction fluids. The strength of the dynamic seals which are formed by the screw elements with opposing pitch and the pressure which these seals are able to withstand depend on the geometry of the screw element and on the Theological properties (the way in which the material deforms under the influence of a force) of the material with which the extruder is filled. The Theological properties can be adjusted b mixing the starting material with a plasticizer or lubricant. For example, with caraway seeds as the starting material, the addition of a small amount of water has a substantial influence on the properties in the seals.